We don't need a WW2 sized threat to justify the sorts of ships that I am talking about. We just need one nation that we are having a disagreement over some islands to our north who happens to have 3 or 4 reasonably modern DE subs. We have a couple of those and all that they would have to do is to SAY that they are going to hit Aussie coastal shipping and they would tie up half the ships that we can currently deploy. We can't afford that so we need a cheaper, more numerous solution.

As for the plausibility of PAR's on Corvettes LM didn't design the SPY-1K not to work. I know that using it or a smaller CEAFAR on a Corvette would necessitate using smaller missiles, but I'd guess that that ESSM can tackle a brace of sub launched missiles coming at a convoy. Sea skimmers wouldn't be detected by radar much beyond the ESSM's range anyway so the shorter range probably wouldn't matter. Having a stealthy corvette wouldn't help in that situation either as the targets are the ships that it is escorting which have a RCS the size of a well, bulk freighter.

As for pirates there was an article in the Australian about those today. Apparently we have too small a merchant fleet to be too bothered by them. Also I'm pretty sure that most of the merchant ships routed to Australia don't pass through the worst areas like the Gulf of Aden or the Straits of Malacca. It isn't one of our defence priorities.

1. Fisheries protection and smuggling is.

Our patrol boat fleet has that pretty much covered up north. OPV's like NZ has bought  might be useful for policing the Southern Ocean against toothfishermen and the like, but I'm not sure that the problem is big enough to justify it.

Counterargument-your OPVs are too short-enduranced and under-gunned to perform the mission described in your AO. You need the the blue water hull capability to operate as well as the enforcement tools..One of those tools is the helo and the boat party.

2. ESSM is a fairly big and robust rocket. It actually outperforms ASTER.........significantly. it needs a fairly hefty radar to make full use of its potential. The rockets a small corvette might carry are more akin to RAM or Barak. 

As I have already pointed out LM has already designed a corvette-sized AEGIS.  They didn't do it with RAM or Barak in mind. A 1500 tonne hull could easily handle a couple of 8 packs of ESSM, which would suffice for our purposes. The 1000 tonne Thai Ratanakosin Class Corvette carries the  24 Aspide which is Sparrow-based so it is well and truely do-able.  

Counterargument.

Aspide Mk1/Mk2

ESSM:

Type Medium-range surface-to-air missile

Place of origin  United States

Service history In service February 2004 aboard USS Chafee[citation needed]

Production history Manufacturer Raytheon

Produced September 1998

Specifications

Weight 620 lb (280 kg)

Length 12 ft (3.66 m)

Diameter 10 in (254 mm)

Warhead 66 lb (39 kg) blast-fragmentation

Detonation mechanism Proximity fuze

Engine Mk 143 Mod 0 solid fuel rocket
Operational range 27+ nm (50+ km)

Speed Mach 4+

Guidance system Midcourse datalink
Terminal semi-active radar homing

Launch platform Mk 41 VLS (RIM-162A/B)
Mk 48 VLS (RIM-162C)
Mk 29 box launcher (RIM-162D)

Much bigger rocket is the ESSM-very much so at 18% by mass and 22% by volume. The ESSM 2 if it goes to the the 255 mm kill-body barrel will be 30% and 43% larger respectively. You are discussing a missile that right now outperforms a Standard I and if ot becomes fatter will approach an early Standard II in performance flyouts.

A corvette-sized radar even the LM one doesn't see far enough (4x) to exploit those rockets' flyouts. Even Aspide out-flys its radar coverage on some of its current platforms. There is such a thing as too much rocket for the radar to use

Match the resources to the mission. I also always find it interesting and amusing that the RAN is historically pro-active instead of reactive to the bluster and threats Australia's neighbors made.

The RAN's most important mission is to be able to defend Australia's interests against credible threat's in the case of war. Being pro-active under those circumstances isn't just a quaint characteristic of RAN brass and the DOD, it is a good idea. It would take far longer for us to build a class of Corvettes that can undertake this sort of mission than it would for Indonesia to buy a dozen sub launched Exocets for their 209's and whatever sub-launched missile fits into the Kilo's tubes. Quite apart from that our Navy can't currently adequately defend itself against TNI's missile armed FAC in the littorals. So much for proactivity.

Counterargument, you send Mister Aircraft (pick a type) with NAShM or some other anti-ship missile to pay the FACs a pro-active visit. Your standoff and systems integration is an order of magnitude greater than his as are your CM/CCM capabilities.

I'm not afraid of two or three diesel electric Russian boats or older 209s except as they can lay mines and torpedo the occasional freighter. The Kilo is also tech restricted to Russian sub launched AShMs Those are not very good. You can fox them. Heck Australia invents many of the CMs that the US uses to fox AShMs.

You need to not underestimate the importance oif EW at sea or in the air in this discussion. Its not the side with the biggest gun that wins anymore. Its the side with the best electronics.